This invention relates to methods and apparatus for cleaning semiconductor wafers and the like.
In the fabrication of semiconductor wafers used in making a variety of semiconductor circuit devices, the importance of minimizing contamination on the wafers has been recognized since the early days of the industry. However, as the end product devices have become more and more miniaturized and complex, the cleanliness requirements have become increasingly more stringent so that the devices will function properly. With the reduced size of the devices, a contaminant occupies an increased percentage of the available space for current elements, and hence cleanliness of the materials becomes far more critical.
As the devices become more complex, they also become more valuable, such that unsatisfactory products represent a very significant loss of revenue. A cassette load of large diameter wafers may have an end process value of as much as a million dollars. Also, there is the cost incident to unusable end products that might arise as a result of the discovery of unsatisfactory semiconductor devices after their combination with other components.
In addition to the foregoing cost there are the major expenses associated with the cleaning processes themselves. One of the major capital expenditure is in the cost of cleaning and drying equipment and associated plumbing, heating and cooling equipment, robotic wafer handling apparatus, computerized control equipment, apparatus for storing and disposing cleaning solutions, and the clean room space required for the apparatus. Of course, there is the cost of the cleaning solutions and the cost of heating, cooling and filtering the solutions, as well as the cost of storing and disposing of them. In view of environmental concerns and regulations, the cost of disposal of certain materials can be greater than the cost of the material being discarded.
Still the most common system for cleaning semiconductor wafers utilizes a series of tanks containing the necessary cleaning solutions, with the tanks being positioned in a xe2x80x9cwet benchxe2x80x9d in a clean room. A batch of semiconductor wafers is moved in sequence through the series of tanks, usually by means of computer controlled automated apparatus. A major concern with this type system is that of contamination occurring as the batch of wafers is transferred from one tank to another. Also of significance is the possible contamination introduced by the handling apparatus itself. Further, whenever wafers are moved, there is the risk of damage to the wafers due to mishandling.
Another system, rather than utilizing tanks which are open to the surrounding clean room, utilizes a full, continuous flow of cleaning solutions through a pipe-like construction. A supposed advantage of that system is that by keeping the wafers immersed in cleaning fluids throughout the process, the risk of contaminants being on the wafers is decreased. The effectiveness of this system, however, is somewhat controversial, and the apparatus is relatively expensive to purchase and to operate.
Still the most commonly used cleaning solutions are those developed by RCA many years ago employing hydrogen peroxide chemistry, particularly those referred to as xe2x80x9cstandard clean 1xe2x80x9d or xe2x80x9cSC-1xe2x80x9d and xe2x80x9cstandard clean 2xe2x80x9d or xe2x80x9cSC-2.xe2x80x9d SC-1 typically comprises ammonium hydroxide, hydrogen peroxide and deionized water in the following ratios: 1 NH4OH:1 H2O2:5 H2O. SC-2 usually comprises 6 H2O:1 H2O2:1 Hcl. Typically wafers are immersed in these solutions for 10 minutes at 25-80xc2x0 C. for each solution. Intermediate and final rinses of deionized water are used between chemical steps. If the wafers are particularly contaminated, there is an initial cleaning step utilizing a solution known as xe2x80x9cCarosxe2x80x9d or xe2x80x9cPirhana,xe2x80x9d typically comprised of H2SO4 and H2O2 in ratios varying from 2-5:1. Following the use of Pirhana there is frequently an additional etching step employing DHF (dilute hydrofluoric acid).
While those solutions contain the most commonly used chemicals and those are the most common ratios, solutions with other ingredients and solutions with different ratios have been utilized, including some with relatively dilute solutions of the active ingredients. A modified SC-1 mixture of 0.01 NH4OH:1H2O2:5H2O has been reported to help reduce surface roughening.
An additional technique for loosening particles, is that referred to as megasonic cleaning. In this technique, highly effective non-contact scrubbing action on both front and back side surfaces of the wafers is achieved by extremely high-frequency sonic energy, while the wafers are submerged in liquid. By utilizing the megasonic system, with standard cleaning solutions, films and adsorbed contaminants are removed at the same time that particles are being removed by the megasonic energy. Sonic waves of 850-900 Khz are generated by an array of piezoelectric transducers. Particles ranging in size from several micrometers down to about 0.2 micrometers have been efficiently removed with input power densities of 25 watts/in. Megasonic cleaning systems are available from VERTEQ, INC., assignee of the present invention.
As noted above, because of the advances in the miniaturization and functions of semiconductor circuit devices, improved semiconductor cleaning techniques are highly desirable. Some of the goals or industry needs are to reduce particulate levels to less than 0.1 micron, to reduce defect density levels to less than 0.001 particles/cm2, and to reduce surface metallic contamination levels to 1E8 atoms/cm2. In addition, it is desirable to eliminate chemical cross-contamination from transfer of cassettes from one tank to another, as with traditional systems. Further, an important goal is to control the cleaning processes to prevent or minimize surface microroughness of the finished product. Another goal is to reduce the high cost of ownership associated with wet chemistry processing, which includes the cost of cleaning solutions and their disposal, and many other elements. It is, of course, always desirable to lower the initial cost of equipment and to improve the reliability of the equipment.
The two priority applications referred to above contain claims directed to improved processes employing dilute chemistry, and also claims to the improved apparatus and method of use. The claims of the present application are directed to the apparatus and method of using the apparatus. However, the processes concerning the dilute chemistries are also summarized here so as to better understand the apparatus. Briefly stated, the improved semiconductor cleaning processes disclosed herein utilize highly dilute cleaning solutions different from the commonly used SC-1 and SC-2 solutions, and megasonic energy is applied to the wafers selectively during the cleaning steps and during the rinsing steps. The cleaning and rinsing steps are preferably all performed in a single tank or without moving the wafers, utilizing a combination of continuous flow, quick dump and spray rinse techniques.
Such a process accomplishes the aforementioned objectives of improved cleanliness and reduced costs of equipment and costs of ownership. The use of highly dilute cleaning solutions not only reduces the cost of materials required, but also eliminates the need for the handling of toxic materials in that most of the highly dilute solutions can be disposed of without special handling equipment or techniques. It has also been found that the excellent cleaning results are obtained in a much shorter time than that required for the conventional process of moving a batch of wafers through a sequence of cleaning tanks. Rather than using conventional SC-1 and SC-2 chemistries using highly dilute cleaning solutions enables the effective, high purity chemicals to be injected to an incoming water flow, thereby eliminating other previously used batch mixing techniques for such materials. Further, the use of dilute solutions reduces the volume of rinsing water and the amount of time required for rinsing.
The preferred cleaning system of the invention which may be referred to as xe2x80x9cVcS,xe2x80x9d includes several steps or cycles, some of which have optional aspects, and each of which includes the use of a preferred dilute solution. Cycle 1, which may be referred as Vc1, includes a solution comprising 300-600 H2O:2 H2O2:1 NH4OH and about 14 ppm of a suitable surfactant. A second cycle, which may be referred as Vc2 employs a highly diluted buffered oxide etch (BOE). In a third cycle, which may be identified as Vc3, a solution is employed having 1000 H2O:5 H2O2:1 NH4OH and 20 ppm surfactant. In a fourth optional cycle, Vc4, a solution is employed having 1,000 parts H2O to 1 part Hcl. These unique solutions are provided at certain preferred temperatures, in a particular sequence and manner, and also, megasonic energy is applied in certain stages.
In a preferred sequence of the Vc1 cycle, a tank containing the wafers to be cleaned is quickly filled with hot deionized water. In one example, the tank was filled in about a minute with hot DI water. The wafers are then inserted. As the hot water is entering, cold DI water is also introduced, at a low rate, to carry chemical and to cause tank overflow after the tank is filled. The diluted surfactant and the ammonium hydroxide are injected at the beginning of the cycle at rates that cause them to reach the desired ratios in the solution. Hydrogen peroxide is introduced into the tank after a delay period of about 25 seconds at a rate to cause that component to reach its desired strength in the final solution at the same time as the ammonia. All the chemical additives for the Vc1 solution are in the tank within about a minute and a half, such that the above-mentioned ratios are attained, except to the extent that a minor amount of the chemical is continuously lost due to the continued low flow of cold DI water into the tank and its overflow at the top.
In one form of the invention, each of the chemical is introduced through a manifold into the DI water flow entering the tank. The inflowing DI water and chemical are circulated throughout the lower portion of the tank and then rise as a uniform mixture. In a preferred form of the invention, a diffusing element is positioned below the wafers being cleaned to prevent splashing of incoming liquid and to cause the solution to rise uniformly in a laminar flow pattern within the tank so that all the wafers are treated in a similar manner.
In another form of the invention, each of the chemicals are injected directly into the tank rather than with the DI water. The injection ports are in the lower portion of a side wall of the tank adjacent the water inlet port and below the diffuser. Megasonic energy is applied to the interior of the tank after an initial delay of about 15 seconds, but prior to the insertion of the wafers.
One of the significant advantages of the invention is that it has been found that the wafers only need to be subjected to the cleaning solutions for a short period of time, even though they are very dilute. Thus, after the chemicals have been added in about the first 90 seconds, it is only necessary to continue the flow of cold DI water and the application of megasonic energy for about an additional 90 seconds. This creates a total cleaning cycle time of about 3 minutes.
The tank is then quickly emptied and cool DI water is sprayed onto the wafers and the interior sides of the tank. Simultaneously, the megasonic energy power level is reduced. The tank is then refilled with cold DI water at a high flow rate, while the cold spray continues. Once the level of liquid in the tank covers a megasonic energy transducer array in the lower part of the tank, the megasonic energy is once more applied at full power until the cold DI flow is terminated and the tank is once more dumped. The cold DI spray is interrupted near the end of the cold DI flow, but then is continued when the dump valve is opened. These steps of dumping and rinsing are repeated as needed.
If a diluted hydrofluoric acid (DHF) treatment of the Vc2 cycle is employed, HF in the form of diluted buffered oxide etchant (BOE) is applied to the wafers. This may be accomplished by dumping the tank contents, raising the cassette of wafers, filling the tank and then immersing the wafers in the tank. To prevent streaking, the wafers should be quickly immersed in the HF bath. This solution acts to strip the oxide, removing metals which are less electronegative than silicon. The megasonic energy is not applied during this period of time in that it has been found that the dilute buffered hydrofluoric acid treatment is better without megasonic treatment. Utilizing the megasonic treatment when the wafer surface is hydrophobic tends to cause microroughening of the surface. The wafers are only treated for about a minute before the solution is quickly dumped to a discharge tank or recycling unit.
The above method of controlling HF is one of three techniques disclosed. A second method of utilizing diluted HF is to remove the wafers from the overflowing and megasonically active DI wafer tank and move them to a free-standing recirculated and filtered diluted hydrofluoric acid (DHF) tank for the time required to remove the oxide. At the end of the time period the wafers are quickly transferred back to an overflowing DI water bath without megasonic energy. This technique is best suited for more concentrated HF solutions where greater than 100 Axc2x0 of oxide is required to be removed from the surface.
Another technique which is preferred for HF-last applications or applications in which only native oxides of 20-30 angstroms (Axc2x0) need to be removed is to inject small amounts of HF or BOE into the cold DI water stream to create the desired concentration of HF. At the end of the etch period (approximately 2 minutes), the HF injector is turned off and high flow DI water rinse is begun, removing the chemistry from the wafers without exposing them to an air interface. This reduces the particulate contamination associated with this interface.
A combination of the latter two techniques is also possible when moderate to large amounts of oxide ( greater than 300 Axc2x0) need to be removed and it is desired to terminate the DHF sequence with the injection techniques to reduce contamination levels for HF-last requirements. This approach would be accomplished by starting the etch in the separate recirculated and filtered HF tank for a sufficient time to remove all but the last 50-75 Axc2x0 of oxide. The wafers would then be quickly transferred to the overflowing DBF mixture already prepared prior to the transfer. The remaining oxide would then be removed at a much lower rate. The process is terminated by stopping injection of HF. High flow DI water then replaces the chemistry in situ. The overflowing chemistry and DI water are collected in the overflow weir and directed to a dedicated HF waste treatment drain. Once the dilution is adequate, the diverter valve can be switched to the normal plenum drain.
Rinse DI water is applied to the tank and allowed to overflow for about 5 minutes, without spray or dump. In some situations, the quick dump procedure is followed to rinse the wafers more quickly. The wafers are then dried for HF last application. Or if the Vc3 cycle is desired, surfactant is injected into the cold DI line; and after 15-30 seconds, a small amount of hydrogen peroxide is injected into the cold DI stream. During this phase, cold DI is flowing continuously through the tank at high rate, causing overflow. The flow of cold DI water, the H2O2, and the surfactant is then stopped for about 1 minute.
Hot water flow then starts with surfactant, followed by the introduction of hydrogen peroxide through the cold DI line. Ammonium hydroxide is introduced once the bath temperature reaches 40-45xc2x0 C. and is continued for about 15 seconds. The flow of hydrogen peroxide and surfactant continues for about a minute and is stopped about when the NH4OH stops, while the LOW-FLOW COLD DI water continues.
In a final cleaning step of the Vc3 cycle, hot DI water at a high flow rate is once more injected into the tank, causing a high rate of overflow at the same time that surfactant is introduced into the cold DI line. After a short delay, additional ammonium hydroxide and hydrogen peroxide are introduced for about a minute. Once the temperature reaches 50-55xc2x0 C., the megasonic energy is once more applied. After the introduction of the hot DI and the chemicals, only cold flow DI continues for a short period, while the application of megasonic energy continues. The tank is then subjected to a series of dump-and-rinse cycles, with the megasonic energy applied when the transducer area is covered with liquid.
If the Vc4 cycle is desired, hot DI water is introduced to displace cold DI water in the tank. After the hot water has been flowing for a short period, a small amount of Hcl is injected into the cold DI flow line. The megasonic energy is applied throughout this period of time. The tank is then subjected to a final series of dump and rinse cycles. The wafers are then ready to be dried.
The above-described process provides surprisingly good results compared to conventional xe2x80x9cwet bench,xe2x80x9d multiple tank techniques. The particle removal capability of the diluted chemical components is remarkable, as is the minimization of undesirable metals and surface microroughness. Prior systems which were thought to be adequate were usually not even tested for the presence of particles below 0.3 microns. This process is capable of removing particles below 0.11xcexc. In addition, the overall time required for the entire operation is much less than that required by conventional techniques. Also, less chemicals and less water are employed, resulting in reduced costs. In addition, the equipment is less expensive and the space required is reduced.
Another major advantage of the present invention is that the chemicals are used in such dilute quantities that they can, except for the HF, simply be sent to drain along with the DI water. The amount of chemical in the water may be less than that which is flushed to drain when cleaning a tank having had a more concentrated chemical component. Less chemical means less contaminants, particularly since the chemicals are the major cause of undesirable metals. Another advantage of the system is that all cleaning steps can be performed in a single tank.
Although possibly unnecessary, current environmental considerations require that even very dilute HF be stored for special handling for disposal or else to be recycled. Thus, when the liquids used during the Vc2 cycle are drained from the cleaning tank, they are transferred to a special tank rather than down a regular drain. One preferred alternative is to provide one or more reservoirs that enable the Vc2 liquid to be used again within the same tank. This can be done with a lower reservoir and an upper reservoir arranged so that the process tank can be quickly dumped and refilled, employing large valves in the side walls of the process tank.